[sswee]

Your problem with the dye time is proof of the need to keep notes on the parameters of your runs. What was the average tank temp when the parts dyed well as opposed to when they started to take longer to get the same color? Coating thickness and pore size effect the dye process. With all the variables, it's sometimes difficult to distinguish a problem with the coating from a problem with the dye. If you are still able to get the same color from the dye albeit a much longer dye time and you use the dye at the same temp then your problem should be from the coating. What has changed? I've run my tank for right at two years and had no problems with acid concentration. Most problems have been connections, temps, different material grades or miscalculating SA.
If you know or can figure out what has changed, it'll help diagnose the problem.
SS